Photolithography is a well-known technique for applying patterns to the surface of a workpiece, such as a circuit pattern to a semiconductor chip or wafer. This technique has the advantage of being able to faithfully reproduce small and intricate patterns. Traditional photolithography involves applying electromagnetic radiation (light) to a mask having openings formed therein (transmission mask) such that the light that passes through the openings is applied to a region on the surface of the workpiece that is coated with a radiation-sensitive substance, e.g., a photoresist. The mask pattern is reproduced on the surface of the workpiece by removing either the exposed or unexposed photoresist. The capabilities of conventional photolithographic techniques have been severely challenged by the need for circuitry of increasing density requiring higher resolution features. This is particularly true for advanced or next generation lithography where the goal is to produce circuits whose critical dimensions are below 0.1 μm. The demand for smaller feature sizes has inexorably driven the wavelength of radiation needed to produce the desired pattern to ever-shorter wavelengths, i.e., toward extreme ultraviolet (EUV) or soft x-ray radiation. Two frequently used sources of such radiation are a laser-produced plasma (LPP) and synchrotron radiation. Laser plasma sources are as bright as their more expensive synchrotron counterparts and are better suited to a small laboratory or commercial environment. The plasma, produced by directing a laser at a target composed of a condensed gas such as krypton or xenon, as it expands through a supersonic nozzle into a vacuum chamber, has been shown to produce a stable source of extreme ultraviolet (EUV) radiation, i.e., light whose wavelength in the range 3.5–15 nm. The generation of EUV radiation by means of a LPP has been described in U.S. Pat. No. 5,577,092 “Cluster Beam Targets for Laser Plasma Extreme Ultraviolet and Soft X-rays”. Thus, a laser produced plasma (LPP) is well suited for producing the EUV radiation required for next-generation lithography tools and the use of a LPP for EUV lithography has been described in U.S. Pat. No. 6,031,598 “Extreme Ultraviolet Lithography Machine”.
While generation of EUV radiation by means of a LPP reduces or eliminates many of the problems associated with other sources of EUV radiation, such as production of damaging atomic and particulate debris, there are still significant problems associated with LPP radiation generation. Chief among these is the cost, primarily driven by the cost of the laser diodes required to pump the lasers. Moreover, the laser pump poses severe technological problems.
In an attempt to overcome the difficulties associated with LPP radiation sources, Silfvast, in U.S. Pat. No. 5,499,282 “Efficient Narrow Spectral Width Soft X-ray Discharge Source”, describes a pulsed electrical capillary discharge source. The electrical discharge source employs a pulsed high voltage, high current electric discharge in a low pressure gas to excite a plasma confined within a capillary bore region. Any gas that can be ionized to generate a plasma that produces radiation at the appropriate wavelength can be used. For generating EUV radiation and soft x-rays xenon is the most desirable species.
FIG. 1 is a cross-sectional view of a typical electrical discharge source 100 that comprises generally an insulating disc 110, typically fabricated from a ceramic material, having an axial capillary bore 115, a front electrode 120 and a rear electrode 130, disposed on either side of insulating disc 110, each having an aperture aligned with capillary bore 115. In operation, a gas, i.e., Xe gas, flows through capillary bore 115 from rear electrode 130 to front electrode 120. A high voltage, high current electrical pulse is established across the front and rear electrodes by power supply 140 causing electrons in the gas to be accelerated and to collide with and excite gaseous atoms causing them to emit radiation. In the case of Xe gas the radiation is extreme ultraviolet (EUV) radiation at about 13.5 nm. However, this discharge source ejects significant amounts of debris eroded from the capillary bore and electrodes.
The intense plasma generated in the capillary bore tends to heat the capillary walls and, depending upon the material used, causes the surface of the capillary bore either to vaporize or to repeatedly melt and solidify. Furthermore, significant stresses are introduced near the surface of the capillary bore by the intense thermal gradients generated during the discharge cycle. The combination of these stresses and changes in the physical state of the capillary bore surface cause material to break away from the surface and generate debris. It is this debris that can coat and erode the surface of proximate optical components used to collect EUV light, thereby severely affecting their reflectance and reducing their efficiency.
Debris generation remains one of the most significant impediments to the successful development of electrical capillary discharge sources for EUV lithography and various modifications of the basic electrical discharge source design have been proposed to block debris from reaching critical optical features. As illustrated in FIG. 2, these prior art modifications generally employ physical means, such as reconfiguration of front electrode 120. While providing a direct blocking path for debris travel, debris emitted in an angular distribution from the capillary can still escape. Moreover, these modifications also intercept a significant portion of the generated EUV radiation, permitting only collection of that radiation that is emitted in an angular direction from the capillary.